The problem of scaling in and on process equipment used in industrial processes and particularly in those processes having an alkaline process stream is very well known. The scales present a significant problem when they build up on the surface of process equipment and cause a loss in the heat transfer coefficient. Thus, additional heat may be required to be provided to the evaporator equipment in these processes resulting in added cost.
An example of such an industrial process having an alkaline process stream is the Kraft recovery process for manufacturing paper which has been known for over 100 years and is eloquently described in many texts on the subject (see G. A. Smook “Handbook for Pulp and paper technologists”, 3rd Edition). More recently the development of closed loop cycles in kraft paper mills has resulted in an increase in scaling problems in process equipment due to the build up of aluminum and silicon in the system as is described by P. N. Wannamaker and W. J. Frederick in “Application of solubility data to predicting the accumulation of aluminum and silicon in alkaline pulp mills”, Minimum Effluent Mills Symposium, 1996, p 303. It has, therefore, been a well recognized need to provide a method and compositions for inhibiting the formation of aluminosilicate scales in kraft pulp mills. U.S. Pat. No. 5,409,571 describes the use of terpolymers of maleic acid, acrylic acid and hypophosphorous acid as scale inhibitor for kraft pulp mills. This type of polymer is shown to be effective against calcium carbonate scales but has not been shown to be effective for aluminosilicate scales.
High Level Nuclear Waste (HLNW) facilities process radioactive-rich solid and liquid wastes in order to minimize waste volume and immobilize the hazardous material for long term storage. HLNW treatment is currently performed via two processes; one process is performed under acidic conditions and one under alkaline conditions. Under alkaline processing conditions, sodium aluminosilicate scale growth is a significant problem during the pretreatment stage, prior to waste vitrification.
Within the pretreatment facility, the waste is evaporated, filtered, ion exchanged and further evaporated. During evaporation, aluminosilicate scales can form on the surfaces of the evaporator walls and heating surfaces. Furthermore, transfer pipes can also become blocked due to the buildup of these scales and precipitates necessitating closure for maintenance.
The pretreated HLNW wastes go to vitrification facilities. HLNW waste goes into a melter preparation vessel where silica and other glass-forming materials are added. The mixture is then heated and the molten mixture is then poured into large stainless steel containers, cooled and moved into temporary storage until a permanent storage location is selected.
From the vitrification unit operation, a portion of the Si-containing glass-forming materials are recycled back into the evaporator unit (during pretreatment). The dissolved aluminum, in the form of sodium aluminate, and sodium silicate species react slowly in solution to form complex hydrated sodium aluminosilicate species. Among these species are families of amorphous aluminosilicates (aluminosilicate hydrogel), zeolites, sodalites, and cancrinites collectively known as “sodium aluminosilicate”. These nuclear waste streams also contain high concentrations (up to 2M for each ion) of nitrate and nitrite ions, and very high concentrations (up to 16M in some sections of the tank) of OH− ions. These factors greatly enhance the rate of formation of aluminosilicate scale. As a result, sodium aluminosilicate scale formed has a low solubility in the alkaline HLNW liquor.
Also, sodium aluminosilicate scale is considered to be an undesirable HLNW product due to the incorporation of radioactive lanthanides and actinides into the aluminosilicate scale cage structures and coprecipitation of sodium diuranate. (Peterson, R. A. and Pierce, R. A., (2000), Sodium diuranate and sodium aluminosilicate precipitation testing results, WSRC-TR-2000-00156, Westinghouse Savannah River Company, Aiken, S.C.). It is therefore, desirable for HLNW facilities to minimize the volume of HLNW's including those resulting from aluminosilicate scales. Thus, it can be seen that, sodium aluminosilicate scale growth has a significant negative economic and operational impact on the treatment of nuclear wastes.
Therefore, it would be desirable to provide a solution to the sodium aluminosilicate scaling problem in the nuclear waste evaporators.
Attempts to solve the aforementioned problems have lead to limited success see Wilmarth and coworkers (Wilmarth, W. R., Mills, J. T. and Dukes, V. H., (2005), Removal of silicon from high-level waste streams via ferric flocculation, Separation Sci. Technol., 40, 1-11. These authors have examined the use of ferric nitrate to remove Si from solution in the form of a ferric precipitate, in order to reduce or eliminate the formation of aluminosilicate scale. Although this approach has some merit, there is still the disposal of the high-level ferric precipitate to deal with and an additional filtration unit operation is required. Also, W. R. Wilmarth and J. T. Mills “Results of Aluminosilicate Inhibitor Testing”, WSRC-TR-2001-00230 have proposed using low molecular weight compounds as scale inhibitors for HLNW's but have found none to be satisfactory.
Thus there is a need for an economical and effective method for reducing aluminosilicate scale buildup on equipment used in industrial processes where such buildup is a problem as for example, the Kraft pulp paper process and in nuclear waste treatment streams.